Question

I’m looking for a way to inspect the contents of a HttpServletResponse to sign them with a MD5 hash.

The pseudocode might look like this

process(Response response, Request request){

defaultProcessingFor(response,request);

dispatcher.handle(response,request);

// Here I want to read the contents of the Response object (now filled with data) to create a MD5 hash with them and add it to a header.
}

Is that possible?

Answer 1

Yes, that’s possible. You need to decorate the response with help of HttpServletResponseWrapper wherein you replace the ServletOutputStream with a custom implementation which writes the bytes to both the MD5 digest and the “original” outputstream. Finally provide an accessor to obtain the final MD5 sum.

Update I just for fun played a bit round it, here’s a kickoff example:

The response wrapper:

public class MD5ServletResponse extends HttpServletResponseWrapper {

    private final MD5ServletOutputStream output;
    private final PrintWriter writer;

    public MD5ServletResponse(HttpServletResponse response) throws IOException {
        super(response);
        output = new MD5ServletOutputStream(response.getOutputStream());
        writer = new PrintWriter(output, true);
    }

    public PrintWriter getWriter() throws IOException {
        return writer;
    }

    public ServletOutputStream getOutputStream() throws IOException {
        return output;
    }

    public byte[] getHash() {
        return output.getHash();
    }

}

The MD5 outputstream:

public class MD5ServletOutputStream extends ServletOutputStream {

    private final ServletOutputStream output;
    private final MessageDigest md5;

    {
        try {
            md5 = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            throw new ExceptionInInitializerError(e);
        }
    }

    public MD5ServletOutputStream(ServletOutputStream output) {
        this.output = output;
    }

    public void write(int i) throws IOException {
        byte[] b = { (byte) i };
        md5.update(b);
        output.write(b, 0, 1);
    }

    public byte[] getHash() {
        return md5.digest();
    }

}

How to use it:

// Wrap original response with it:
MD5ServletResponse md5response = new MD5ServletResponse(response);

// Now just use md5response instead or response, e.g.:
dispatcher.handle(request, md5response);

// Then get the hash, e.g.:
byte[] hash = md5response.getHash();
StringBuilder hashAsHexString = new StringBuilder(hash.length * 2);
for (byte b : hash) {
    hashAsHexString.append(String.format("%02x", b));
}
System.out.println(hashAsHexString); // Example af28cb895a479397f12083d1419d34e7.